Voluntary muscle movement is caused by electrical impulses which originate in the somato-motor cortex of the brain. A neuron in the somato-motor cortex sends electrical signals to a motor neuron in the spinal cord, which in turn sends electrical signals which stimulate the contraction of muscle fibers, producing movement. All of the muscle fibers which are stimulated by a given motor neuron are called a “motor unit.” Each muscle fiber exhibits an electrical potential across its cell membrane, which changes when the muscle contracts.
In electromyography (EMG), the difference in potential on the surface of the skin is measured between the center and the ends of a muscle, which gives a measure of the number of contracting muscle fibers. EMG is regularly used to diagnose a variety of medical conditions in patients, as well as in healthy subjects for research on muscle function.
In stroke patients with damage to their somato-motor cortex, electrical signals are not generated for one or more muscles or parts of muscles, or do not reach those muscles, and normal contraction of those muscles is impossible. Often, residual EMG signals, too weak or too spread out to cause the muscles to contract, are still detectable.
Neuromuscular electrical stimulation (NMES) is used to produce contraction of a muscle which cannot contract normally in a stroke patient. NMES may stop spasticity in a muscle, and may prevent the muscle from atrophying. It is also known to turn NMES of a single muscle on or off in response to residual EMG signals detected from that muscle, thereby allowing the muscle to contract under the control of the patient.
Wireless implantable electronic stimulators have been described, for example in: U.S. Pat. Nos. 5,193,539, 5,193,540, 5,312,439, 5,324,316, 5,405,367, PCT Publication WO 98/37926, PCT WO 98/43700, PCT Publication, WO 98/43701 Oct. 8, 1998, U.S. Pat. No. 6,051,017, U.S. application Ser. No. 09/077,662 and in an article “Micromodular Implants to Provide Electrical Stimulation of Paralyzed Muscles and Limbs”, by Cameron, et al., published in IEEE Transactions on Biomedical Engineering, Vol. 44, No. 9, pages 781-790. The disclosures of all of these references are incorporated herein by reference.
The NESS H200 is an external worn stimulator for stimulating muscles in the forearm and hand, for rehabilitation. Some details are provided in U.S. Pat. No. 6,829,510, the disclosure of which is incorporated herein by reference.